PROJECT SUMMARY There is a growing body of evidence from both laboratory and field studies that disrupted circadian function, particularly decreased amplitude and stability of rhythmic behaviors represent significant risk factors for cardiometabolic disease (CMD) in humans. The exciting evidence of the ubiquity of circadian clocks in all tissues and their critical role in metabolism, not only opens up new avenues for understanding the mechanistic interactions between central and peripheral clocks in cardiometabolic disease pathogenesis, but also to develop therapeutic interventions to re-establish synchrony between central and peripheral clocks with each other and with the external physical and social environments. Feeding has been shown to synchronize clocks in peripheral tissues. Animal studies have demonstrated that restricting feeding to the active period decreases CMD risk, while in humans decreased caloric intake in the evening is associated with a lower body mass index (BMI). The amplitude of melatonin can be considered a marker of robustness of central circadian function, but melatonin also has physiological effects beyond circadian regulation throughout the body. Recent observations have demonstrated that having a low melatonin level is a risk factor for incident diabetes and hypertension independent of sleep duration. Together, the evidence suggests that strategies aimed at synchronizing feeding behavior and enhancing the nocturnal melatonin signal can positively impact cardiometabolic function. We propose to take an innovative approach that combines the recent data on the role of feed/fast patterns on clock regulated metabolic activity and the reemergence of scientific interest of the central and peripheral effects of melatonin on cardiometabolic function to elucidate the physiological and molecular mechanisms that underlie the relationship between circadian dysregulation and obesity associated CMD risk. This will be accomplished by strengthening the amplitude of circadian metabolic signals via extended overnight fasting (EOF) and enhancement of nocturnal circadian signaling with exogenous melatonin in overweight and obese middle aged and older adults. In addition, this study will provide crucial information regarding the importance of circadian timing for the design of future clinical trials on CMD in overweight and obese adults. This is a critical time in the lifespan when circadian based strategies for prevention and treatment are most likely to have the greatest impact on CMD risk. This project will enroll 100 adults (40-65 years) to participate in a parallel (4 arm intervention) placebo controlled study to determine whether a six- week program of extended overnight fasting (EOF) of 12- 14 hours and/or low dose (3 mg) melatonin administration will enhance circadian amplitude and enhance cardiometabolic function, as well as to evaluate the potential beneficial effects of a regimen that combines both approaches. The results from this study will demonstrate novel mechanistically based approaches for maintaining and improving circadian-metabolic health during a critical time in the lifespan when there is a rapid increase in the prevalence of CMD.